The present invention relates to an apparatus for performing electrical resistance seam welding, the apparatus being of the type including at least one electrode arrangement intended for rolling relative to pieces to be welded and provided with a pressure member which does not contact the pieces being welded and is connected to the welding current supply. This pressure member is made of a material having a high electrical conductivity and which is a good thermal conductor, e.g. copper. There also is provided a contact member in the form of an endless, rotating element which is intended to roll relative to the pieces being welded and which can be moved and pressed against the pieces being welded by the pressure member.
It is known that the resistance welding of certain materials, particularly iron sheets having metallic coatings of, for example, tin, zinc, lead, etc. presents considerable difficulties because the copper base electrodes usually employed form, under the influence of the welding temperatures and the contact pressure, alloys with the material of the coating of the pieces being welded and thus become contaminated. This alloy formation, which cannot be, or can only very slightly be, influenced in the course of the process, changes the welding parameters, particularly the material characteristics of the electrode material which influence the passage of current and heat, and thus interferes with the formation of the welds so that the formation of a uniform weld seam is no longer assured. The mechanical properties of the electrodes, such as hardness, heat resistance, geometric configuration, etc. may also be changed by the above-mentioned processes, likewise adversely influencing the welding parameters and the electrode lifetimes.
Swiss Pat. No. 370,175 describes an apparatus of the above-mentioned type in which the stated difficulties are to be avoided in that the contact member of the electrode is provided in the form of a wire, preferably a copper wire, which is placed around a pressure roller and is driven so that the welding location continuously receives fresh, i.e., not yet contaminated, wire and while the wire which has been contaminated by the welding process is removed. According to one embodiment of this prior art apparatus, the wire is guided in an endless loop around the pressure roller and a deflection roller and after leaving the welding location is passes through a cleaning device so that it can be used several times.
However, this has drawbacks in practice. The welding pressure and welding temperature impart considerable changes in cross section to the copper wire and, moreover, sufficient cleaning of the wire from the surface contamination caused by the alloy formation would be possible only by the removal of considerable amounts of material from the wire cross section. These influences that produce cross section changes would permit only a few revolutions of the endless wire loop while the welding parameters would change according to the changes in cross section. Even approximately sufficient liftimes cannot be realized with such an apparatus.
Another variation of the prior apparatus which has therefore found acceptance in practice is one in which the copper wire is unwound from a supply roll and, after passing through the welding location, it is would on a take-up reel to then be removed from the apparatus and melted, for example. As described in German Auslegeschrift [Published patent application] No. 1,565,803, it is also possible to pass the wire through the welding location twice, once at the upper side and once at the underside. This method is presently being widely used to provide rolled weld seams on sheet metals, having very thin metal coatings, particularly those used in the canning industry.
The drawback of this process, however, is that the copper wire is consumed material which must continuously be replenished and which is used up after one or two passes. For continuously welding larger quantities of material it is necessary to provide continuous replenishment of wire supply reels to the welding apparatus and to remove the take-up reels holding the used copper wire. Each change of reels requires an interruption in the operation so that, in the end, this prior art metod constitutes a discontinuous process. Since the wire, with respect to its weight, accepts less than 1% of the coating metal, e.g. tin, during the welding process, an unduly large quantity of copper wire must be made available.
Further drawbacks are that the relatively soft copper wire is deformed by the welding pressure so that during its second passage it no longer has the same cross section as in the first passage, and complicated and expensive mechanisms are required for guiding, routing, winding and unwinding the copper wire, for compensating changes in length occurring during welding and possibly also for reworking the cross section of the copper wire.
It is also known to use composite electrodes to spot weld metal coated sheets. These composite electrodes are formed of a major portion of copper and an insert or contact member of tungsten or some other metal or alloy which does not alloy, or alloys only poorly, with the coating material, e.g. as described in German Offenlegungsschrift [Laid-open application] No. 1,914,424, and which is intended to contact the material to be welded. In this way, electrode contamination due to alloy formation can be reduced considerably. However, considerable difficulties to arise due to the different coefficients of thermal expansion of the electrode materials which at the welding temperatures lead to considerable thermal stresses in the electrodes, to breaking, and loosening of the connection between pressing member and contact member and thus to a change in the current passage and heat dissipation characteristics which, in the end, result again in uncontrollable changes in the welding parameters and/or reduced electrode lifetimes.
These difficulties, which are already encountered in spot welding electrodes, are much greater for compound electrodes for welding rolled seams consisting, for example, of a copper roll and a circumferential coating of tungsten or the like, because for the latter electrodes a perfect connection with more uniform current and heat flow properties must be maintained over the entire circumference of the electrode and on the welding material subjected to the unrolling movement. Moreover, the required lifetimes here are substantially greater.
It has not yet been possible to find a satisfactory design for a compound electrode for welding rolled seams.